The basic principles of exhaust gas recirculation (EGR) are well known. After a properly operating engine has performed work on the combination of fuel and inlet air in its combustion chamber, the engine exhausts the remaining gas from the engine cylinder. An EGR system allows a portion of these exhaust gases to flow back into the engine cylinder. This recirculation of gases into the engine cylinder may be used during positive power operation, and/or during engine braking cycles to provide significant benefits.
During positive power operation, an EGR system is primarily used to improve engine emissions. During engine positive power, one or more intake valves may be opened to admit fuel and air from the atmosphere, which contains the oxygen required to burn the fuel in the cylinder. The air, however, also contains a large quantity of nitrogen. The high temperature found within the engine cylinder causes the nitrogen to react with any unused oxygen and form nitrogen oxides (NOx). Nitrogen oxides are one of the main pollutants emitted by diesel engines. The recirculated gases provided by an EGR system have already been used by the engine and contain only a small amount of oxygen. By mixing these gases with fresh air, the amount of oxygen entering the engine may be reduced and fewer nitrogen oxides may be formed. In addition, the recirculated gases may have the effect of lowering the combustion temperature in the engine cylinder below the point at which nitrogen combines with oxygen to form NOx. As a result, EGR systems may work to reduce the amount of NOx produced and to improve engine emissions. Current environmental standards for diesel engines, as well as proposed regulations, in the United States and other countries indicate that the need for improved emissions will only become more important in the future.
Generally, there are two types of EGR systems, internal and external. Many conventional EGR systems are external systems, which recirculate the gases from the exhaust manifold to the intake port through external piping. Many of these systems cause exhaust gas to recirculate through the external piping by opening a normally closed EGR control valve in the piping during the intake stroke.
For example, U.S. Pat. No. 5,617,726 (Apr. 8, 1997) to Sheridan et al. and assigned to Cummins Engine Co., Inc discloses an EGR system which includes an EGR line connecting the exhaust line and intake line of the engine, cooler means for cooling the recirculated portion of the exhaust gases, a bypass line for bypassing the cooler means, and valve means for directing the flow of the recirculated portion of the exhaust gases.
U.S. Pat. No. 4,147,141 (Apr. 3, 1979) to Nagano and assigned to Toyota discloses an EGR system which includes an EGR pipe for interconnecting an exhaust pipe and an intake pipe of an engine, an EGR cooler being positioned along the EGR pipe, a bypass pipe being arranged parallel to the EGR cooler, a selection valve for controlling the flow of exhaust gas through the cooler bypass, and an EGR valve mounted on the EGR pipe for controlling the flow of exhaust gas through the EGR pipe.
Many external EGR systems require several additional components, such as, external piping, bypass lines, and related cooling mechanisms, in order for the system to operate properly. These additional components, however, may significantly increase the cost of the vehicle, and may increase the space required for the system, creating packaging and manufacturing concerns. In addition, the combination of exhaust gas and moisture in the external piping may expedite the corrosion of system components, leading to reliability issues. Various embodiments of the present invention may be simpler, less expensive, and more reliable than many known external EGR systems that require these additional components.
Many conventional internal EGR systems provide EGR by taking exhaust gas into the combustion chamber through an open exhaust valve during the intake stroke. Without proper control, this technique may create performance problems due to the reduced amount of oxygen in the cylinder. Even though a satisfactory combustion situation may be obtained in the light-load operating range in which there is naturally an excess of air, problems may develop in the high-load operating ranges in which the proportion of air with respect to fuel is low (lean). These combustion conditions may create sub-optimal power and, in addition, may produce black smoke with large amounts of soot.
It is, therefore, desired to provide systems and methods for providing internal EGR events without the power and emissions problems associated with many conventional EGR systems. An advantage of various embodiments of the present invention is that they may provide the necessary control to avoid these pitfalls when actuating an exhaust valve during the intake stroke. In addition, various embodiments of the present invention may provide EGR by actuating one or more intake valves during the exhaust stroke.
An EGR system may also be used to optimize retarding power during engine braking operation by controlling the pressure and temperature in the exhaust manifold and engine cylinder. During engine braking, one or more exhaust valves may be selectively opened to convert, at least temporarily, the engine into an air compressor. In doing so, the engine develops retarding horsepower to help slow the vehicle down. This can provide the operator with increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle. By controlling the pressure and temperature in the engine using EGR, the level of braking may be optimized at various operating conditions.
EGR may be provided with a compression release type engine brake and/or a bleeder brake. The operation of a compression-release type engine brake, or retarder, is well known. As a piston travels upward during its compression stroke, the gases that are trapped in the cylinder are compressed. The compressed gases oppose the upward motion of the piston. During engine braking operation, as the piston approaches the top dead center (TDC), at least one exhaust valve is opened to release the compressed gases in the cylinder to the exhaust manifold, preventing the energy stored in the compressed gases from being returned to the engine on the subsequent expansion down-stroke. In doing so, the engine develops retarding power to help slow the vehicle down. An example of a prior art compression release engine brake is provided by the disclosure of the Cummins, U.S. Pat. No. 3,220,392 (November 1965), which is incorporated herein by reference.
The operation of a bleeder type engine brake has also long been known. During engine braking, in addition to the normal exhaust valve lift, the exhaust valve(s) may be held slightly open continuously throughout the remaining engine cycle (full-cycle bleeder brake) or during a portion of the cycle (partial-cycle bleeder brake). The primary difference between a partial-cycle bleeder brake and a full-cycle bleeder brake is that the former does not have exhaust valve lift during most of the intake stroke. An example of a system and method utilizing a bleeder type engine brake is provided by the disclosure of Assignee's U.S. Pat. No. 6,594,996 (Jul. 22, 2003), a copy of which is incorporated herein by reference.
Many known EGR systems are not useful with existing engine brake systems. Many of these systems: (1) are incompatible with compression release brakes, bleeder brakes, or both; and/or (2) require significant modifications to the existing engine in order for the EGR and braking systems to work properly together. One advantage of various embodiments of the present invention is that they may be used in conjunction with compression release braking systems and/or bleeder braking systems, and require little or no modification to the existing engine in order for the two systems to operate properly.
An EGR system may incorporate additional features to improve performance. Embodiments of the present invention may incorporate, for example, valve catch devices, valve lift clipping mechanisms, EGR lash, selective hydraulic ratios, and reset mechanisms to improve the reliability and performance of the system.
Additional advantages of the invention are set forth, in part, in the description which follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.